Method for operating a dishwashing machine and maintaining the active oxygen content in the wash tank

ABSTRACT

A dishwashing machine includes a washing tank for receiving wash water, and a detergent from a detergent dispensing system. A separate dosing system includes a tank for storing bleach, and a pump for supplying bleach from the bleach tank to the washing tank in measured quantities at predetermined times to maintain a desired active oxygen content therein over a complete cycle of operation.

BACKGROUND

1.0 Field Of The Invention

This invention relates generally to dishwashing machines, and moreparticularly to methods and apparatus for supplying the washing tank ofa dishwashing machine with chemical substances.

2.0 Discussion Of Related Art

In institutional dishwashing machines, with which the present inventionis particularly concerned, fresh water is supplied to washing tank--alsoknown as the dosing tank--in the clear-rinse phase. The correspondingquantity of detergent--controlled through conductivity--is subsequentlyadded as required. Alkaline detergents of the described type used inpractice contain active chlorine as their bleach component, mostly inthe form of liquid chlorine bleach in the case of liquid detergents orin the form of di- and trichloroisocyanurates in the case of powder-formdetergents.

The use of active chlorine as bleach in dishwashing machines isecologically objectionable. As a result, efforts are being made to useenvironment-friendly bleaches. Considerable difficulties have beenencountered in this regard. At the present time, only active oxygen isavailable as an economically reasonable alternative to active chlorine.However, this bleaching agent cannot be formulated with the generallyhighly alkaline liquid detergents.

Although solid active oxygen carriers can be incorporated in powder-formalkaline detergents, corresponding detergent formulations quickly losetheir bleaching effect after flushing into the washing tank of thedishwashing machine because the active oxygen content of the rinseliquor quickly diminishes--often at around 40 to 60% per hour--duringmachine stoppage times. When a new rinse cycle begins, insufficientactive oxygen is available in the washing tank so that cleaningperformance can be unsatisfactory at the beginning of the rinse cycle.

EP-A-0 195 619 describes a method in which rinse cycles are carried outwith a substance containing active oxygen after one or more wash cyclesin a dishwashing machine comprising a spray unit.

In addition, U.S. Pat. No. 4,373,863 describes an apparatus forsupplying the washing tank of a dishwashing machine with chemicalsubstances which consist of a detergent dosing system for supplyingdetergent and a separate dosing system comprising a pump and a storagecontainer for supplying another substance.

3.0 Summary Of The Invention

The problem addressed by the present invention was to enable activeoxygen to be used as bleach in conjunction with an alkaline liquiddetergent and to eliminate any significant reduction in the activeoxygen content in the washing tank during pauses in dosing, even where ableach-containing powder-form detergent is used. For the methodmentioned at the beginning, the solution provided by the invention ischaracterized in that, where active oxygen is used as the bleachcomponent, the active oxygen content in the washing tank is maintainedduring pauses in the dosing of detergent by maintenance dosing of thebleach. The maintenance dosing preferably takes place in individualdosing strokes.

Accordingly, one embodiment of the invention relates to maintenancedosing of the bleach during the stoppage times of the dishwashingmachine between two successive rinse cycles. The effect of thismaintenance dosing is that fresh bleach containing active oxygen entersthe washing tank as the active oxygen content diminishes. Accordingly,the washing tank of the dishwashing machine is always kept in readinessfor a new rinse phase and is therefore able immediately to supplydishwashing detergent containing sufficient bleach at the beginning ofthe rinse phase.

Another embodiment of the invention is characterized in that, where adetergent, particularly a powder-form detergent, basically containingsufficient bleach for immediate rinsing is used, bleach continues to beintroduced into the washing tank as the active oxygen content decreasesduring the machine stoppage times or pauses in the dosing of detergent.

In addition to this maintenance dosing regime, a parallel bleach dosingregime is preferably provided in cases where a bleach-free detergent,particularly a liquid detergent, is used. According to another aspect ofthe invention, this means that, where a bleach-free detergent is used,bleach is introduced into the washing tank commensurately with detergentconsumption during the rinse phases and commensurately with thereduction in the active oxygen content during the stoppage phases or thepauses in the dosing of detergent. Accordingly, the parallel dosingregime mentioned provides for the use of active oxygen carriers, such asfor example hydrogen superoxide (H₂ O₂) and/or peracetic acid solution,which cannot be formulated with the usual bleach-free alkalinedetergents.

According to the foregoing, liquid or powder-form detergent is onlyintroduced or flushed in during certain dosing times in dependence uponthe conductivity control or the like, even during the rinse phases. Ifthe corresponding intervals or the pauses in the dosing of detergentbecome so long that the active oxygen content decreases to the extentwhere the cleaning performance of the next rinse phase is impaired,bleach continues to be introduced, again by maintenance dosing,commensurately with the reduction in the active oxygen content, even inintermediate or standard pauses in the parallel dosing occurring duringthe rinse phases.

In another embodiment of the invention, the bleach input capacity of themaintenance dosing is optimized by determination of the active oxygencontent. Once the rate of decomposition of the active oxygen is known,it can be sufficient if the bleach continues to be introduced at certaintime intervals. In this connection, it is preferred to commenceintroduction of the bleach by maintenance dosing after the decompositionof about 20% of the active oxygen present.

In an apparatus for supplying the washing tank of a dishwashing machinewith chemical substances, consisting of a detergent dosing system forsupplying detergent and a separate dosing system comprising a pump and astorage container for supplying another substance, the solution providedby the invention is characterized in that the other dosing system is ableach dosing system which has an operational mode for maintenancedosing which is activated during the pauses in the dosing of detergent.

The detergent dosing system may be designed in the usual way, i.e. as adosing pump in the case of a liquid detergent or as a fresh water orliquor flush-in system in the case of a powder-form detergent. Accordingto the invention, therefore, the only addition is the bleach dosingsystem separate from the detergent dosing system. This provides forparallel dosing at the same time as dosing of the bleach-free detergentand provides for maintenance dosing during the stoppage times of thedishwashing machine or the pauses in the dosing of detergent so that theconsumption of active oxygen during such periods is compensated.

This auxiliary or maintenance dosing is necessary if, as is usually thecase, the dishwashing machine is supplied with detergent via a controlsystem which does not take the bleach content into account. This isbecause, in contrast to active oxygen, the concentration or quantity ofdetergent remains constant during the stoppage phases. Without themaintenance dosing of bleach, only detergent would be introduced intothe washing tank at the beginning of the rinse phase commensurately withthe inflow of fresh water. Accordingly, there would be a deficit ofactive oxygen and the outcome of the dishwashing process would beunsatisfactory, at least in regard to bleachable soil. According to theinvention, these problems are overcome by the addition of a bleachdosing system.

According to the foregoing, there are in particular two possibleembodiments of the invention, namely: (1) either the bleach dosingsystem has only one operating mode for maintenance dosing, i.e. whenbleach-containing detergent is present, or (2) the bleach dosing systemhas an additional operating mode--functioning in parallel with thedetergent dosing system--for parallel dosing when the detergent dosingsystem is switched on in cases where a bleach-free detergent is used.Accordingly, both bleach-containing and bleach-free detergent can beused, bleach only having to be introduced once during the entire periodof operation of the dishwashing machine and, in the other case, onlyduring the stoppage times between two rinse phases or only during dosingof the detergent. Essentially, parallel and maintenance dosing onlydiffer in the bleach dose, i.e. in the quantity of active oxygenintroduced into the washing tank per unit of time.

In another embodiment of the invention, a frequency-controlledperistaltic pump may be used for the parallel or maintenance dosingregime of the bleach dosing system. A pump of this type can be operatedwith a number of pump strokes per unit of time corresponding to thedecomposition of active oxygen. For maintenance dosing, relatively fewpump strokes are sufficient whereas a much larger number of pump strokesis necessary for parallel dosing where a bleach-free detergent is used.Accordingly, the associated frequency control of the pump preferablycomprises a first control range for maintenance dosing and, optionally,a second control range for the rinse phase of the dishwashingmachine--when the detergent dosing system is switched on--with aconsiderably increased input capacity in relation to the dose of thefirst control range.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail in the following withreference to the accompanying drawings, in which similar items areidentified by the same reference designation, wherein:

FIG. 1 is a block circuit diagram of a parallel and maintenance dosingsystem for bleaches of one embodiment of the invention.

FIGS. 2A, 2B, and 2C are each associated timing diagrams of the paralleland maintenance dosing system for bleaches in an institutionaldishwashing machine of FIG. 1, respectively.

FIG. 3 is a block circuit diagram of a maintenance dosing system in adishwashing machine supplied with powder-form detergent containingactive oxygen in another embodiment of the invention.

FIGS. 4A, 4B and 4C are each associated timing diagrams of the detergentand active-substance supply system shown in FIG. 3, respectively.

FIG. 1 shows the liquid detergent and bleach supply section associatedwith an institutional dishwashing machine. A standard dosing pump 1 forliquid detergent pumps liquid detergent 2 from a liquid detergent tank30, for example under the control of a conductivity measuring system,through a pipe 3 to a washing tank (not shown) of a dishwashing machine.Provided in parallel with the liquid detergent dosing pump 1 is aperistaltic pump 4 which delivers bleach 5 from a tank 32 containingactive oxygen through a pipe 6 to the washing tank mentioned above. Thisadditional parallel and maintenance dosing system for liquid activesubstances can be controlled by internal and external electroniccircuitry in such a way that the functional diagram shown in FIG. 2 isobtained.

FIGS. 2A, 2B, and 2C show various "on" and "off" states (1/0) as afunction of the time t in three lines 7, 8 and 9, respectively. Theoperational state of the dishwashing machine is symbolized in theuppermost line denoted by the reference 7 in FIG. 2A. The rinse phasesor "on" states are denoted by the reference 10 while the stoppage phasesor "off" states are denoted by the reference 11. The normal dosing ofdetergent, i.e. the operational state of the liquid detergent dosingpump 1, is represented by the second line denoted by the reference 8 inFIG. 2B. In the illustrated embodiment, it is assumed that the dosingpump 1 is activated twice during a machine "on" state 10 in dependenceupon the conductivity of the detergent 2 accommodated in the washingtank of the dishwashing machine. The corresponding two detergent dosingphases are denoted by the references 12 and 13 in FIG. 2B. Anintermediate interval is denoted by the reference 14 while the mainpause corresponding to the "off" state 11 of the dishwashing machine isdenoted by the reference 15. The additional dosing of active substance,i.e. the operational state of the peristaltic pump 4, is symbolized inthe third line denoted by the reference 9 in FIG. 2C.

As shown in FIG. 2C, parallel dosing 16 with a rapid succession ofindividual pump strokes (shown as narrow pulses 34) of the peristalticpump 4 is provided during the detergent dosing phases 12, 13 of thedetergent dosing pump I. By contrast, the work of the peristaltic pump 4is greatly reduced during the pauses 14, 15 in the operation of thedishwashing machine. In the corresponding maintenance dosing regime 17,there are considerably fewer pump strokes 34 per unit of time thanduring the parallel dosing regime in the rinse phases 10. It can be seenthat liquid detergent 2 does not continue to be dosed during thestoppage phases 11 of the dishwashing machine. Nor is this necessarybecause there is no reduction in the quantity of detergent solution.Only the reduction in the active oxygen content during the stoppagephase 11 is compensated by the decelerated follow-up or maintenancedosing regime 17 in these phases. It can also be favorable to provideindividual pump strokes 34 of a maintenance dosing regime 18 duringrelatively long intervals 14 between the rinse phases 10 of thedishwashing machine in order to compensate the continuing reduction inactive oxygen during those intervals.

If the dishwashing detergent 2 already contains active oxygen, as ispossible in the case of powder-form detergents, the parallel dosingregime according to the invention generally does not have to beprovided, instead the maintenance dosing regime is sufficient. As shownin FIG. 3, the normal dosing of powder-form detergent 19 containingactive oxygen takes place in a funnel 20 by fresh water or solutionflush-in 21 and a pipe 22 leading to the washing tank of the dishwashingmachine (arrowed direction).

In addition, as in FIG. 1, a bleach dosing system with a peristalticpump 4 is provided in FIG. 3. The peristaltic pump 4 is designed to pumpbleach 5 from a bleach tank 32 through a pipe 6 to the washing tank ofthe dishwashing machine. In contrast to FIG. 1, the peristaltic pump 4shown in FIG. 3 only operates during the stoppage phases 11 of thedishwashing machine because, in the normal detergent dosing regime, thebleach 5 is dosed together with the detergent during the rinse phases10. Only during an interval 14 between two normal dosings 12 and 13, asshown in FIGS. 4B and 4C, might it be favorable--similarly to the caseof FIGS. 2B and 2C--to provide one or more of the dosing strokes 34which form the maintenance dosing regime 18 during the stoppage phase 11(see FIG. 4A) of the dishwashing machine.

A modification of a frequency-controlled peristaltic pump mayadvantageously be used to operate the maintenance dosing regime and theparallel dosing regime, if any. To this end, the frequency controlsystem has two control ranges, namely a first range I for the paralleldosing regime 16 (see FIG. 2C) with a pump 4 output range of 8 to 290ml/min. and a second range II for the maintenance dosing regime 17, 18(see FIG. 4C) with a pump 4 output range of 1.5 to 3.5 ml/min. These twocontrol ranges are designed to be externally selected to enablereadjustments to be made in dependence upon the result of thedishwashing process.

LIST OF REFERENCE NUMERALS

1=dosing pump for liquid detergent

2=liquid detergent

3=liquid detergent pipe

4=peristaltic pump

5=bleach

6=bleach pipe

7=uppermost line (FIG. 2)

8=second line

9=third line

10=rinse phase

11=stoppage phase

12=detergent dosing time (1)

13=detergent dosing time (1)

14=interval (1)

15=main pause (1)

16=parallel dosing

17=maintenance dosing

18=maintenance dosing

19=powder-form detergent

20=funnel

21=flush-in of detergent by wash liquor

22 =pipe

What is claimed is:
 1. A method for operating a dishwashing machinecomprising a wash tank in which a detergent assisted or complemented inits effect by a bleach is introduced into the wash tank, wherein saidmethod comprises the steps of:including an active oxygen carrier in saidbleach; introducing a plurality of individual doses of detergent atdifferent times into said wash tank, during predetermined phases ofoperation to maintain a desired level of detergent in said wash tank;and introducing doses of said bleach containing an active oxygen carrierinto said wash tank at times that detergent is not being added to saidwash tank, for maintaining the active oxygen content in said wash tankabove a predetermined minimum level over rinse and stoppage phases ofoperation of said dishwashing machine.
 2. A method as claimed in claim1, wherein said oxygen maintaining step further includes saidmaintenance dosing being provided in individual dosing strokes.
 3. Amethod as claimed in claim 2, wherein when a powder-form detergentcontaining sufficient bleach for immediate rinsing is used, said methodfurther includes the step of continuing to introduce bleach into thewashing tank as the active oxygen content decreases during machiningstoppage phases or pauses in the dosing of detergent.
 4. A method asclaimed in claim 2, wherein when a bleach-free liquid detergent is used,said method further includes the step of introducing said bleach intothe washing tank commensurately with detergent consumption during rinsephases and commensurately with the reduction in the active oxygencontent during stoppage phases or pauses in the dosing of detergent. 5.A method as claimed in claim 2, wherein said maintenance step includesusing hydrogen superoxide as the active oxygen carrier.
 6. A method asclaimed in claim 2, wherein said maintenance step includes usingperacetic acid solution as the active oxygen carrier.
 7. A method asclaimed in claim 2, further including the steps of:determining the rateof decomposition of the active oxygen in said wash tank; and calculatingfrom the rate of decomposition of the active oxygen, time intervals atwhich doses of said bleach must be added to said wash tank to maintainthe active oxygen content thereof above a predetermined level.
 8. Amethod as claimed in claim 1, wherein when a powder-form detergentcontaining sufficient bleach for immediate rinsing is used, said methodfurther includes the step of continuing to introduce bleach into thewash tank as the active oxygen content decreases during machiningstoppage phases or pauses in the dosing of detergent.
 9. A method asclaimed in claim 8, further including the steps of:determining the rateof decomposition of the active oxygen in said wash tank; and calculatingfrom the rate of decomposition of the active oxygen, time intervals atwhich doses of said bleach must be added to said wash tank to maintainthe active oxygen content thereof above a predetermined level.
 10. Amethod as claimed in claim 1, wherein when a bleach-free liquiddetergent is used, said method further includes the step of introducingsaid bleach in to the washing tank commensurately with detergentconsumption during rinse phases, and commensurately with the reductionin the active oxygen content during stoppage phases or pauses in thedosing of detergent.
 11. A method as claimed in claim 10, furtherincluding the steps of:determining the rate of decomposition of theactive oxygen in said wash tank; and calculating from the rate ofdecomposition of the active oxygen, time intervals at which doses ofsaid bleach must be added to said wash tank to maintain the activeoxygen content thereof above a predetermined level.
 12. A method asclaimed in claim 1, further including the steps of:determining the rateof decomposition of the active oxygen in said wash tank; and calculatingfrom the rate of decomposition of the active oxygen, time intervals atwhich doses of said bleach must be added to said wash tank to maintainthe active oxygen content thereof above a predetermined level.
 13. Amethod as claimed in claim 12, wherein the step of maintenance dosing ofthe bleach is commenced after a reduction of around 20% in the originalactive oxygen content.
 14. An apparatus as claimed in claim 13, furtherincluding a frequency control system for said pump having means forproviding a range of control over bleach doses for the maintenancedosing regime.
 15. A method as claimed in claim 1, wherein saidmaintenance step includes using either one of hydrogen superoxide orperacetic acid solution as the active oxygen carrier.
 16. A method asclaimed in claim 1, wherein the step of maintenance dosing of the bleachis commenced after a reduction of around 20% in the original activeoxygen content.
 17. An apparatus for supplying the wash tank of adishwashing machine with chemical substances, comprising a detergentdosing system for supplying detergent, a separate dosing systemincluding a pump, and a storage container for supplying anothersubstance, wherein said separate dosing system includes a bleach dosingsystem which includes means providing an operational mode formaintenance dosing of said bleach, which is activated during pauses inthe dosing of detergent, for maintaining the active oxygen content insaid wash tank over rinse and stoppage phases of operation of saiddishwashing machine.
 18. An apparatus as claimed in claim 17, whereinsaid bleach dosing system further includes means providing anoperational mode functioning in parallel with said detergent dosingsystem for providing a parallel dosing regime when the detergent dosingsystem is on.
 19. An apparatus as claimed in claim 18, wherein said pumpincludes a frequency-controlled peristaltic pump for pumping the bleach.20. An apparatus as claimed in claim 17, wherein said pump includes afrequency-controlled peristaltic pump for pumping the bleach.
 21. Anapparatus as claimed in claim 20, further including a frequency controlsystem for said pump having means for providing a range of control overbleach doses for the maintenance dosing regime.